This invention relates generally to a camshaft drive for an internal combustion engine and more particularly to a drive arrangement for a pair of adjacently arranged parallel camshafts.
In a drive of the type that is described in German Patent Application No. P 33 09 376.8-13, toothed gearing is used which provides a driving connection from the crankshaft to the camshaft via several intermediate stages. Such toothed gearings are required for some applications for reasons of safety (for example, aircraft engines), but cause loud noise emissions and are quite costly to manufacture. Therefore, camshafts are conventionally driven by means of toothed belts which wind around a toothed belt wheel of the crankshaft and a toothed belt wheel of the camshaft. Since this toothed belt drive arranged on the front side of the internal combustion engine is at the same time used for driving several accessory apparatuses, the toothed belt is relatively long and tends to stretch during operation. This danger exists especially in the case of high-performance engines which, during acceleration, cause such a high straining and stretching of the belt that occasionally the belt may skip a tooth. The resulting change of the control time of the valves causes increased fuel consumption and higher emissions of harmful substances.
Accordingly, one objective of the present invention is the provision of a camshaft drive that eliminates unintended changes in the timing of the valves.
Another objective of the present invention is the provision of a camshaft drive that operates at a minimal noise level.
A further objective of the present invention is the provision of a camshaft drive that permits timing changes during the operation of the internal combustion engine.
An even further objective of the present invention is the provision of a camshaft drive that can be manufactured in a cost- effective manner.
These and other objectives of the present invention are attained by the provision of a camshaft drive for first and second parallel adjacent camshafts wherein the first camshaft is driven by a toothed belt driven by the crankshaft of the internal combustion engine. The second camshaft is driven by a chain drive connected with the first camshaft. The relative rotating position of each camshaft can be changed by means of a hydraulic tensioning device engageable with the chain drive. The control time of the valves of the internal combustion engine is adjusted by changing the relative rotating position of the camshafts by means of the tensioning device.
As discussed above, the toothed belt connecting the camshaft with the crankshaft in high performance engines will routinely be stretched and strained during acceleration of the engine. The lengthening of the toothed belt results in a change of the rotating position of the camshaft driven by the toothed belt. The tensioning device of the present invention compensates for the change of the rotating position of the camshaft with respect to the crankshaft which results from this lengthening of the toothed belt. The general type of chain tensioning device employed in the present invention is disclosed in German DE-OS No. 2 249 310. However, that disclosure relates to use of such a tensioning device with a long chain between the crankshaft and the camshaft. In such an arrangement, a shifting of the tensioning element by a certain amount results in a significantly smaller adjustment at the camshaft than in the case of the shorter chain drive between the camshaft and the intermediate shaft of the present invention. Since relatively large adjustments at the camshaft can be made by relatively small shifts of the tensioning elements according to the present invention, a hydraulic tensioning device can be employed which requires very little space for operation. This space advantage is particularly noticeable in an internal combustion engine having a four valve cylinder head and two overhead camshafts.
In accordance with the present invention, one camshaft is driven by the crankshaft by means of a toothed belt and this camshaft in turn drives the other camshaft by means of a chain. The chain drive with a tensioning device may be so small that it comfortably fits into the cylinder head and may be shielded to reduce noise emissions. Also, because it may be positioned in the oil bath, it is continually protected from dirt and abrasion. Advantageously, it is contemplated that the chain drive will be located in the center of the longitudinally extending camshafts. With this arrangement, the camshafts are stressed symmetrically from the center by the torque developed. Resulting torsion is only half of what it would be if the torque was introduced conventionally on the front side of the cylinder head.
Other objects, advantages and novel features of the present invention will become readily apparent when the following detailed description of the preferred embodiment is considered in conjunction with the attached drawings.